Recently, a distance measuring method with ultrasonic waves has been used as an accurate distance measuring method. The method is generating ultrasonic waves from an ultrasonic generator, causing the ultrasonic waves to impinge on an object, detecting ultrasonic waves reflected from the object by ultrasonic microphone, and calculating the distance to the object from the time elapsed between the generation and the detection.
For example, Patent Document 1 discloses an ultrasonic generator in which piezoelectric transducers are attached to a housing. The ultrasonic generator disclosed in Patent Document 1 is configured as an ultrasonic sensor in which a single device serves as both an ultrasonic generator and an ultrasonic microphone. The ultrasonic generator includes, in addition to a first piezoelectric transducer for generating ultrasonic waves, a second piezoelectric transducer vibrating in opposite phase to that of the first piezoelectric transducer with the aim of cancelling unnecessary vibration.
FIG. 8 illustrates an ultrasonic generator (ultrasonic sensor) 500 disclosed in Patent Document 1. The ultrasonic generator 500 has a structure in which a first piezoelectric transducer 102 and a second piezoelectric transducer 103 for cancelling unnecessary vibration, the second piezoelectric transducer 103 vibrating in opposite phase to that of the first piezoelectric transducer 102, are attached to a housing 101. Each of the housing 101, first piezoelectric transducer 102, and second piezoelectric transducer 103 is connected to a lead 104. The space within the housing 101 is filled with a flexible filler 105.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-297219